I'm a fan of the model where in order to get feedback / have it affect your profile and ranking, you have to give feedback first. So whomever gives feedback first it's held in escrow until the other party does it too.

If the buyer is getting a discount or is signing up for a Beta program, you can make the feedback as part of buyer's contract.
If the buyer is purchasing a regular GA product, providing additional discounts, gift or coupons for their feedback is another method. Something to acknowledge the time they are taking, is often successful.